In recent years, silicon-based photomultipliers have been actively developed, and radiation detection devices such as an X-ray computed tomography (CT) apparatus including a photomultiplier have been also developed. In the X-ray CT device, X-rays that have penetrated through a subject are detected, and a sectional image (reconstructed image) of the subject using a CT value corresponding to an X-ray attenuation factor as a pixel value is reconstructed. Specifically, the X-ray attenuation factor at the time when the X-rays pass through a material (subject) varies depending on the type of the material such as a bone or water, so that an internal structure of the subject is visualized by reconstructing the attenuation factor with the cross section of the subject based on projection data obtained by a detector for detecting intensity of the X-rays passing through the subject while rotating around the subject.
In recent years, to grasp the inside of the subject in more detail, X-ray CT devices that calculate density of each material from the projection data have been in practical use. To calculate such density, known is a method of obtaining density images of two materials by projecting images twice by switching a tube voltage of an X-ray tube in two different ways, using a dual energy CT device because the attenuation factor varies depending on the energy and the density of the material even with the same material. The X-rays include photons having various pieces of energy, and energy distribution varies as the tube voltage varies. In the above method, the density images of the two materials are obtained through the following two routes.
(1) Projection data of two types of tube voltages→attenuation factor images of two types of tube voltages→density images of two materials
(2) Projection data of two types of tube voltages→X-ray transmission lengths of two types of materials→density images of two materials
An image (reconstructed image) having, as a pixel value, a linear attenuation coefficient that is an attenuation factor per unit length of the X-ray or a CT value that is a relative value of the linear attenuation coefficient of air, water, and the like is referred to as an attenuation factor image, and an image having the density of the material as a pixel value is referred to as a density image. The linear attenuation coefficient is uniquely determined based on the type and the density of the material, and the energy of the photons, and the attenuation factor image assuming specific energy can be synthesized from the obtained density of the two types of materials. The attenuation factor image is referred to as a monochromatic image. Contrast of the material of interest can be improved by adjusting the energy for synthesizing the monochromatic image.